A boiling water reactor normally comprises an external, substantially cylindrical, vertical container referred to as a reactor vessel, in the lower part of which a substantially cylindrical vertical moderator vessel is arranged. The moderator tank comprises a core of fuel rods. Between the outer wall of the moderator tank and the inner wall of the reactor vessel, there is an annular space referred to as a downcomer. The reactor vessel is partially filled with a coolant (water) for cooling a core of fuel rods arranged in the moderator tank.
During operation of the reactor, that is, during nuclear fission, the water starts boiling when it has reached to approximately one-fourth of the core. The steam thus formed is separated from the water at the upper part of the reactor vessel, partly in steam separators and partly in steam dryers arranged to separate the last moisture residues in the steam before it flows out of the reactor vessel. The separated water flows down into the downcomer. To replace the water which is taken out of the reactor vessel in the form of steam, the reactor vessel is supplied with water via a feedwater inlet. Thus, the downcomer contains a mixture of incoming feedwater and water which is separated from the steam in the steam separators and the steam dryers.
In the downcomer at the bottom of the reactor vessel, main recirculation pumps of plug-in type are arranged for recirculation of water from the downcomer and up through the core for continuous cooling of the fuel rods. The main recirculation pumps normally consist of vertical wet asynchronous machines operating in water under pressure.
The adjustable recirculation flow of the coolant is utilized for controlling the output power from the reactor in that an increased coolant flow, in addition to an increased cooling of the fuel rods, also results in an increased power production (increased neutron generation) in the fuel rods. As a result of the thermal inertia in the fuel rods, the time constant for increased power production in the fuel rods differs from the time constant for the corresponding increase of the cooling requirement. If the supply of energy to one or more main recirculation pumps is disturbed, the cooling and the power production are interrupted instantaneously whereas the surface temperature of the fuel rods rises since the fuel rods contain a decay power in the form of thermal energy which is not yet exhausted.
During normal operation of the reactor, the fuel rods are surrounded by a coolant film. At too rapid a reduction of the coolant flow, the decay power which is stored in the fuel rods will result in a brief overheating thereof. In those cases where this overheating leads to the heat flux from a fuel rods becoming very great in relation to the coolant flow, there may be a risk of dryout occurring, that is, the coolant film becomes so thin that it is unable to hold together. The coolant film is broken up and dry wall portions are formed, which locally leads to a considerably deteriorated thermal transmittance between the fuel rod and the coolant with an ensuing greatly increased surface temperature of the fuel rod. The increased surface temperature may lead to damage with serious consequences arising on the fuel rods, or to a shortening of the service life thereof.
To secure against dryout, the power output from the fuel rod is limited such that a margin with respect to dryout in case of transients in the coolant flow is obtained. This margin, referred to as dryout margin, means that the fuel cannot be utilized as efficiently as would otherwise be possible. Therefore, from the point of view of fuel economy, it is desirable to minimize the dryout margin. One of the dimensioning factors for the dryout margin is disturbance of the coolant flow as a result of line power loss for shorter or longer periods.
The dryout margin in the case of disturbances of the energy supply to the main recirculation pumps is dependent on how fast the main recirculation pumps unroll, that is, on the time rate of change of the pump speed. The time rate of change is determined by the kinetic energy of the pump, that is, the inertia in the drive system of the main recirculation pump. The unroll time is thus dimensioning for the power output from the fuel rods.
To increase the unroll time it is desirable to increase the inertia in the drive system of the main recirculation pumps. Because of the design of the reactor, the space for pumps and motors is limited, which gives a pump/motor design which is relatively long and narrow with a limited moment of inertia. The limited space in the reactor vessel means that it is not possible to increase the dimension of the main recirculation pump to obtain increased inertia. The space in the reactor vessel only allows increased inertia by replacing material in certain parts of the main recirculation pump by heavier material. However, this is not sufficient to obtain the desired inertia.
For driving a main recirculation pump, it is known to use a drive system comprising a pump motor for driving the pump and a rotating frequency converter for feeding the pump, the frequency converter comprising a motor which via a hydraulic coupling is connected to a generator. The rotating frequency converter is electrically connected between a supply alternating voltage network and the pump motor. The rotating frequency converter may be provided with a flywheel for increasing the inertia in the drive system and hence limiting the unroll speed of the main recirculation pump. The disadvantage of the rotating frequency converter is that it limits the speed of action when controlling the power of the reactor. The rotating frequency converter cannot maintain the speed of the main recirculation pump in case of a voltage loss but only extend the unroll time thereof. A poor efficiency is a general disadvantage for hydraulic couplings.
Another way of extending the unroll time is to connect in the drive system, in series with the rotating motor-generator equipment (without hydraulic coupling), a static frequency converter, the static frequency converter comprising power electronics components in the form of a rectifier connected to a d.c. intermediate link which, in turn, is connected to an inverter for feeding the pump motor. The static frequency converter is fed from the rotating motor-generator equipment which, in turn, is fed from an alternating voltage network. In this way, the drive system is supplied with inertia via the rotating motor-generator equipment whereas the control speed is made possible by the static frequency converter. The disadvantage of this method of extending the unroll time is that modern conventional static frequency converters of pulse-width modulated type are sensitive to voltage deviations in the supply to the rectifier. In those cases where the rectifier is supplied from the generator of the rotating frequency converter, it is difficult to maintain the voltage from the generator when this unrolls. Older static frequency converters are less sensitive to voltage deviations, so the method described above is adapted to these older models. Further, the method requires two machines, namely a motor and a generator. The generator must be of a synchronous machine type, which is more expensive and less reliable than a simpler and more robust asynchronous machine. The network must be able to manage direct start of the flywheel, which limits the maximally possible inertia in the system. In addition, the motor and the generator must be dimensioned for the rated power of the main recirculation pump. Further, the rotating motor-generator equipment has a relatively large service requirement compared with the static frequency converter.
The introduction of inertia in the drive system for a main recirculation pump is of advantage in the event of loss of line power but of disadvantage as regards the speed of action for control of the power production of the reactor. The requirement for control speed in the reactor has increased and, therefore, drive systems comprising only rotating frequency converters are no longer used in new designs.
In a boiling water reactor, normally between four and ten main recirculation pumps are arranged. Only events which lead to unroll of the majority of these pumps give rise to dryout.
One object of the invention is to achieve, for a given, total main recirculation pump dimension, an increased inertia in the drive system thereof for maintaining the speed of the main recirculation pump in the event of loss of supply voltage from the network.